Steve Johnson complains about his chattering wipers...

I have had similar problems. I twisted the wiper arms (as per Bentley) with
some success. But the best solution was fitting "aerofoil" blades. Thes
are rigid plastic holders with channels to "create a downforce" when air
flows through them. Well I don't know if that works but the rubber blade
and the holder are good quality and fit tightly to the arm. They didn't
chatter, and they wiped well.
I bought 18" ones, but I think the next time I'll look into 20", for more
coverage.

Years ago I had a summer job in northern B.C. and flew around a bit in a
jet ranger. The mechanic would spray Pledge on the perspex and the airflow
would move the water off the windshield just as you describe with Rain-X.
However, I don't suggest Pledge on a Vanagon windshield, I bet it would
wear off quickly and streak with the wipers.

Alistair